Utilization of ultrasonic measurements for the determination of various properties of a flowing material within an enclosed conduit is now a well developed art. Numerous publications and patents have described a variety of techniques for mounting ultrasonic transducers from the external or internal surfaces of the conduits and transmitting a generated beam through the fluid, receiving it at a second transducer, and measuring variations in transit time, or its reciprocal which is proportional to sound speed c, or attenuation of the signal. These ultrasonic parameters indicate the flow characteristics, quality, temperature, or other characteristics of the fluid. The ultrasonic flow-sensing signal is typically emitted repeatedly or alternately from one of a pair of transducers mounted at opposite ends of a tilted diameter or diagonal across the conduit, with the direction of tilt being in the direction of flow of a material. The time delay difference (upstream time minus downstream time) between the generation of the emitted signals and their reception is a measure of the velocity of the flow. Other characteristics of the material may be determined by measurement of the attenuation of the received signal. In one embodiment a crossed pair of tilted paths are used with an emitting transducer and receiving transducer located at the ends of each of the crossed pairs. Techniques for measuring both the transit times and the attenuations are described in a number of patents and articles. These include particularly:
__________________________________________________________________________ Patent Number Inventor(s) Issue Date __________________________________________________________________________ U.S. Pat. No. 2,746,291 R. C. Swengel May 22, 1956 U.S. Pat. No. 2,755,662 R. C. Swengel July 24, 1956 U.S. Pat. No. 2,874,568 L. A. Petermann February 24, 1959 U.S. Pat. No. 2,959,054 W. Welkowitz November 8, 1960 U.S. Pat. No. 3,050,997 D. B. Lake August 28, 1962 U.S. Pat. No. 3,553,636 J. D. Baird January 5, 1971 U.S. Pat. No. 3,564,912 J. T. Malone et al February 23, 1971 U.S. Pat. No. 3,636,754 L. C. Lynnworth et al January 25, 1972 U.S. Pat. No. 3,575,050 L. C. Lynnworth April 13, 1971 U.S. Pat. No. 3,731,532 A. Courty May 8, 1973 U.S. Pat. No. 3,788,140 Q. C. Turtle January 29, 1974 U.S. Pat. No. 3,906,791 L. C. Lynnworth September 23, 1975 __________________________________________________________________________
In order to produce an ultrasonic beam emitted at an angle to a diameter across the conduit, a variety of specific techniques have been employed. These include the use of externally mounted wedges for generating an angular refracted beam, as well as a variety of internal machining techniques. One limitation on the accuracy of such ultrasonic measurements arises from what is referred to as "short circuit noise". The short circuit noise originates at the emitting transducer and is coherent at the receiving transducer with the ultrasonic wave transmitted through the fluid. Short circuit noise is ultrasonic energy which travels directly through the walls of the conduit from the emitting transducer to the receiving transducer. It is apparent that such energy is not attenuated or otherwise affected by the fluid characteristics within the conduit and hence presents a noise background signal. The effect of this noise is particularly pronounced where ultrasonic waves transmitted through the fluid material pass through a solid-fluid interface, thereby undergoing attenuation due to the acoustic mismatch between these materials. Since the noise signal undergoes no such mismatch its relative amplitude appears to be enhanced. Interference of this noise signal with the ultrasonic energy transmitted through the fluid leads to errors in measuring the arrival time and the amplitude of the received wave. Where the characteristic being measured is the flow velocity, the maximum error in velocity may be shown to be approximately proportional to the reciprocal of the signal to noise ratio and may be as large as 7% when the signal to noise ratio equals ten for typical flow velocities. This type of calculation can be understood with the aid of a vector diagram as discussed in the article "Clamp-On Ultrasonic Flowmeters: Limitations and Remedies", published in Instrumentation Technology (9), 37-44 (September, 1975).
One technique which has been employed in the past to minimize this short circuit noise problem is to mount at least one of the transducers within an acoustically isolating material in the conduit wall. In such an arrangement a cylindrical transducer is mounted within a cylindrical opening in the wall which is larger in diameter than the transducer and the intervening space is filled with an acoustically damping material. Such a configuration provides difficulties, not only in initially mounting the transducer, but also in maintaining precise alignment of the direction of the beam emitted from the transducer, due to phenomena such as creep of the acoustical damping material.
A second, unrelated, problem encountered in the preparation of ultrasonic flow cells for the measurement of velocity and other characteristics of flowing material, arises from the difficulty, in many instances, of providing access to the interior walls of the conduit. When there is no such access, the choice of techniques for generating the appropriate directional ultrasonic beams is substantially limited to configurations which may be machined or installed from outside the conduit, yet such configurations are not entirely satisfactory for all applications.
It is, therefore a primary object of the present invention to minimize the short circuit noise contribution in ultrasonic flow measurements, thereby enhancing the signal to noise ratio.
It is another object of the present invention to provide an ultrasonic measuring apparatus for materials flowing within a conduit enabling easy access to the interior of the conduit for machining or other mounting or directional-beam-controlling techniques.